In recent years, high-refractive-index films have been adopted for adjusting the refractive index of portable displays and building material glasses. A general niobium oxide target as a high-refractive-index material cannot obtain the conductivity of a DC dischargeable target by an atmospheric sintering process, and thus the conductivity of the sintered body is enhanced by reducing the sintered body under high temperature and pressure conditions (see, for example, Patent Literature 1).
It has also been reported that the resistivity decreases as zinc is added to niobium oxide (see, for example, Patent Literature 2).
However, in any case of these methods, since any of these methods are required to employ a hot press method and a huge press mechanism is required for producing a large target, thus these methods are not a realistic process, and the target size is limited to small products. In addition, in the hot press method, sintering is conducted in a reducing atmosphere and thus the quantity of oxygen deficiency in the target tends to increase. For a target with a large quantity of oxygen deficiency, oxygen is required to be introduced as a sputtering gas at the time of sputtering in order to obtain high transmittance and a problem also arises that the film formation rate decreases by the introduction of oxygen.
In addition, a composite oxide sintered body composed of zinc, aluminum, and titanium has been reported as a high-refractive-index target (see, for example, Patent Literature 3). It is said that a composite oxide sintered body exhibiting less arcing occurrence and stable DC discharge performance is obtained from a titanium-containing zinc oxide-based target as well as a high refractive index of 2.0 or more is realized by the titanium-containing zinc oxide-based target. However, titanium has an extremely low film formation rate to be a half or less than that of niobium, which is the same high-refractive-index material as titanium, and there has been a problem that the titanium-containing target exhibits low sputtering productivity.
In recent years, adoption of cylindrical targets to which high power loads can be input and the like also proceed and film formation to which high power, which has not been hitherto assumed, is input is becoming mainstream. Furthermore, a sintered body obtained by mixing niobium oxide or titanium oxide of a high-refractive-index material as described above with zinc oxide is DC dischargeable in a mixed system of a conductive phase composed mainly of zinc oxide and an insulating phase which is a composite oxide of a high-refractive-index material and zinc oxide, but the conductive phase and the insulating phase coexist in this sintered body, and thus there is a problem that the sputtering current concentrates on the zinc oxide of the conductive phase, zinc oxide is reduced, metal zinc having a low melting point is splashed, and particles are generated as well as holes are formed on the target surface.